The present invention relates in general to computer systems, and in particular to peripheral devices used in computer systems. Still more particularly, the present invention relates to a method and system for using analytical color models to automatically, or rapidly manually, generate multi-dimensional color lookup tables.
As computing moves into the multimedia era, the days in which color was an unneeded luxury and monochrome monitors were commonplace, have given way to color image processing being an integral feature used by most applications. As the commercial demand for color grew, so too did the complexity of the hardware and software which was designed in response to that demand. Many color image processing techniques have been developed to deal with the issues raised by the integration of color into the computing paradigm.
Color matching is one such technique, which is used when transferring color images and documents between color devices, such as monitors, scanners and printers. Color matching is a useful interface technique because color devices have different color capabilities, describe color in different terms and operate in different color spaces. For example, a color display monitor in a computer system may create and describe colors in terms of red, green and blue ("rgb") values, and is then said to work in the RGB color space. The rgb values associated with particular colors for this display monitor are device dependent, meaning the rgb values associated with specific colors are particular for that monitor or brand of monitor. Because the rgb values are device dependent, colors displayed on different monitors will probably not be visually identical even for the same rgb input values.
Most printers create and describe colors in device dependent terms differing from those used by monitors. Printers use, for example, cyan, magenta, yellow and black ("cmyk") values to describe colors, and are said to work in the CMYK color space. Again, because the cmyk values are device dependent, colors printed on any given printer will probably not match colors printed on a different printer for the same cmyk values.
Further complicating color matching between color devices is that different color devices have different color capabilities. Every color device, such as a scanner, printer, or monitor, has a range of colors that it can produce. This range of producible colors is known as a gamut. To exemplify the difficulties caused by differing device gamuts, consider monitors and printers. Those skilled in the art will recognize that color display monitors can produce and display hundreds to thousands of colors. Color printers, however, typically have a smaller number of printable colors. Consequently, in most situations the gamut for a color display monitor exceeds the gamut for a color printer. As a result some colors displayed on display monitors cannot be produced by color printers.
Color matching techniques use models to translate colors between devices while trying to maintain the perceived color appearance. For example, suppose that a user creates an image on a monitor. If she or he prints this image without any color matching, the color appearance of the printed image will differ significantly from that of the original. Using a color matching model, this change can be reduced to a perceptionally acceptable level.
Conventionally, two different types of models have been created to simulate printer operation. Empirical models essentially treat a printer as a black box having an image input and an image output. A large number of test color samples are printed. Each color sample which has been printed is then measured to determine that particular printer's response to that test sample input. Using the measured data, one can create a multi-dimensional lookup table which compensates the printer's response so that the electronic image input to the printer results in the desired printed image. However, using empirical models requires anywhere from about a thousand samples to many thousands of samples to provide an accurate lookup table. Accordingly, such a methodology is too complex to be performed by printer users and is usually only done at the factory for an entire production run of printers. Thus, this technique fails to provide users with the ability to create customized compensation values for various paper stock, inks, etc.
Analytical models, on the other hand, attempt to characterize printer response using the fewest number of measurements possible to arrive at functions which can be used to predict a printer's image output for a sufficient number of different image inputs. These types of models have been under development since the 1930's and historically have not been sufficiently accurate to provide perceptionally acceptable color correction. However, a more accurate analytical model has recently been developed and is disclosed in "Modeling the Color of Multi-Colored Half Tones" authored by J. A. Steven Viggiano in the 1990 proceedings of the Technical Association of the Graphic Arts. According to this analytical model, which is described in greater detail below, a printer can be accurately characterized using only, for example, sixty-one measurements. While analytical models provide a characterization of a printer based on many fewer measured test samples than are required for empirical models, the complexity of these models renders compensation calculations for each pixel of each input image too slow for runtime application.
Another way in which conventional systems have attempted to deal with the problems presented by the different responses of printers to the same color input is to provide a color detector inside the printer which can measure the printer's response by sensing the toners on the printer drum. However, this sensing function does not feed back information for further processing and, therefore, these conventional printers can only provide coarse tone reproduction characteristics and cannot correct for hue or colorfulness offset. Moreover, since these conventional printers read from the drum, they are unable to compensate for changes in reproduced color perception which occur due to different types of media and the ways in which the colorants interact with the paper.